The invention pertains to the field of envelope feeders for laser printers.
Envelope feeders for laser printers have been in use for several years now. Envelope feeders function to receive a stack of envelopes and feed them, hopefully one at a time, into the paper entry port of a printer such as a laser printer. Paper trays typically use corner separators, but these types of separators will not work for card stock and envelopes because they damage the corners of the paper with heavier materials.
Different types of feed mechanisms exist for envelope feeders. Some use gravity feed and some use vacuum feed. Others use motor driven compression stacks to compress a stack of envelopes. A prime example of the latter type of prior art of high-capacity, force feed envelope feeders is the JT25-XXXX 250 envelope feeder first introduced by Genesis Technology of Hayward, Calif. about five years ago.
The problem with all envelope feeders is jams. Jams are frequently caused by multiple feeds, i.e., more than one envelope is taken from the stack and fed into the printer thereby jamming the envelope transport mechanism within the printer. Some technology must be used to separate the envelopes from each other so that only one envelope is fed into the printer at any one time. The difficulty with envelopes that does not exist with stacks of paper is that envelopes have pockets and many different manufacturing styles, paper weights, paper types with varying degrees of coefficients of friction and many different characteristics in terms of trapping air. Many envelope feeders such as the JT25-XXXX predecessor to the invention use the friction retard principle. In this technology, a force is put on the stack of envelopes acting as a normal which defines the frictional forces when considered in light of three different coefficients of friction between: (1) a feed roller and the top envelope, (2) the envelopes in the middle of the stack, and (3) the bottom envelope and a cork pad on the bottom plate. The three frictional forces are: F.sub. 2 acting on the interfacial surfaces between envelopes in the middle of the stack, i.e., the force required to pull an envelope out of the middle of the stack, the frictional force F.sub.3 resisting movement of the bottom envelope in contact with a cork bottom plate, i.e., the force to pull the bottom envelope out of the stack, and the frictional force F.sub.1 between the top envelope and a feed roller, i.e., the force to pull the top envelope out of the stack with the roller stationary, i.e., with the roller not rolling. In the preferred embodiment of the invention, the roller is EPDM rubber of hardness 40.+-.5 durometer value, Shore A (available commercially from VIP Rubber). Separation is accomplished by depending upon the differences between the frictional forces. If F.sub.1 is greater than F.sub.2 and F.sub.3 is greater than F.sub.2 and F.sub.1 is greater than F.sub.3, then separation should occur. Since F.sub.1 must be the largest force, the material of the feed roller must be selected carefully and maintained carefully in terms of cleanliness and aging so that F.sub.1 always is the largest of the three forces. Such an approach frequently leads to multiple feeds and is not reliable enough for some users for several reasons. For example, as the number of envelopes in the stack changes, the relative magnitude of the F.sub.2 force changes for each envelope. This is because the normal force on each envelope is different since the stack of envelopes acts like a series of springs coupled together, each offsetting part of the normal force. Also, because the envelopes have different air catching qualities, different glue and flap characteristics, the coefficients of friction vary between envelopes of different types and made by different manufacturers. Thus, this mechanism may work satisfactorily for some envelopes but not for others.
Accordingly a need has arisen for a more reliable high capacity envelope feeder which uses additional mechanism for envelope separation and normal force control to reduce the number of multiple feeds.